The present invention relates generally to ink jet printing systems and similar drop marking systems in which a supply of electrically conductive ink is provided to a nozzle. Energy is applied to drive the nozzle and the ink is forced through the orifice of the nozzle which causes a stream of ink to separate into droplets at a break-off point. The drops are subsequently charged and deflected onto a substrate to be marked. Charging of the stream of droplets is generally accomplished in this art by directing the flight path of the stream either through or in the proximity of a charging electrode or ring.
Setting the location of the break-off point of the ink stream into droplets and the related task of determining the minimum break-off length have always been a problem in the art of ink jet printing. In order to ensure proper operating conditions for consistent printing quality, the location of the break-off point must be well inside of the charging ring. Preferably, the break-off is set near the minimum distance from the nozzle, a point referred to in the art as the foldback point. Typically a human operator must make manual adjustments while visually inspecting the location of the break-off point. Alternatively a single electrode in the vicinity of the stream separation point can be used to give an approximate position of the break-off point. Neither method provided sufficient control of printing quality.
What is desired is a system which more accurately can determine both the minimum break-off point (foldback) and the present location of the break-off point so that a precise measurement of the time of flight from droplet separation to the detection of the droplets at a downstream detector can be made. Utilization of a charging electrode which has many segments, each capable of being selectively charged via a switching controller, satisfies this desire. The segments are disposed along the direction of the flight path of the stream which allows the break-off point to be measured to within the length of one segment. In turn, this permits accurate determinations of the velocity of the droplets because the time of flight and distance are known. It also permits detection of the foldback point.
Accordingly, it is an object of the present invention to provide an ink jet printing system which utilizes a segmented charge tunnel which allows improved detection of the drop formation point and the foldback point.
It is a further object of the invention to provide a segmented charge tunnel which makes it possible to accurately determine the location of the break-off point so that a satisfactory determination of droplet time of flight can be made. Accurate time of flight measurements allow the droplet stream to be more precisely controlled which allows constant drop deflection and better printing quality to be maintained.